In the RPD's main office, you'll need to access the mainframe to retrieve critical data. The mainframe is located in a secure room. You'll need to find a keycard or bypass a security panel. The keycard is usually held by a high-ranking RPD officer in a nearby office, or the security panel requires a short hacking mini-game (interact and follow on-screen prompts).
Once inside the mainframe room, interact with the computer terminal to begin the data download. This process will take a short amount of time, during which you will be attacked by waves of infected and possibly more RPD officers. Set up defensive positions, utilize choke points, and protect the operative downloading the data. Lickers and Hunters are common threats in this section. Once the download is complete, prepare for extraction.